1. Field of the Invention
This invention relates to a method of utilizing a stabilized mask assembly for use in generation of controlled thin film patterns and more particularly to a method which utilizes a stabilized mask assembly for controlling the edge gradient and position of a layer of vapor deposition material deposited onto a substrate from an evaporation source of vapor deposition material.
2. Description of the Prior Art
Utilization of patterns of thin films in microelectronics is well known in the art. Many methods for generating thin film patterns are known in the art including the use of film deposition masks which are commonly used in an evaporation vacuum deposition process.
Typically, a vacuum deposition mask is fabricated using known etching or electroforming techniques for forming a plurality of apertures in a well-defined pattern. In use, the deposition mask having the well-defined pattern is positioned in close proximity to a substrate upon which a thin film of material formed into the pattern of the well-defined pattern is to be deposited. An evaporant vapor from a vapor deposition source passes through the deposition mask and condenses on the substrate in the form of the well-defined pattern of the deposition mask and wherein the edge gradient is affected by the shadowing effect of the mask to form the thin film layer of material.
In certain of the known prior vacuum deposition processes used for the fabrication of certain electrical components, such as for example, fabrication of thin film transducers, thin film hybrid circuits or discrete electrical components such as helical coils, it is necessary to deposit a plurality of thin film layers of materials in various patterns onto a substrate wherein each deposited layer must be accurately located in an exact position on the substrate and in a predetermined relationship to the prior deposited layers on the substrate.
In connection therewith, it is also know in the art that placing a deposition mask relative to a vapor deposition source in a vapor deposition process results in the mask being subjected to heating due to the type of source being used in the vapor deposition process. Typically, the heat radiated to a mask from a resistively heated boat or crucible type of evaporation source is high. In comparison, the use of an electron beam type of evaporation source results in a lower amount of heat being radiated to a deposition mask.
In the known vapor deposition processes, a deposition mask is subject to heating from at least four (4) possible sources; namely, (a) thermal radiation from the vapor source; (b) heat of vaporization released by the evaporant vapor condensing on the mask itself; (c) the heated substrate and substrate holder located proximate to the deposition mask, and (d) the heated mask support.
The known deposition masks have been formed from metal foil and the so-formed deposition masks are susceptible to changes in operating temperatures during the deposition process. Typically, the metal foil deposition masks have a thin cross-section and as a result thereof, exhibit low conductance of heat and low total heat capacity. As such, the metal deposition mask is highly susceptible to temperature changes resulting from variations in (a) thermal emissivity and thermal conductance due to the presence of evaporation material collected on the mask; (b) heat input to the deposition mask due to variations in the rate of condensation of evaporate material onto the deposition mask; and (c) thermal radiation from the source.
All of the above has the net effect of causing variations in the dimensions of the known deposition mask during the deposition process which results in an inability to precisely control registration and accurate dimensions of the resulting thin film deposited layer or layers.
Further as a result of the deposition mask being exposed to heating, it is known in the art that typically the deposition mask itself expands, the exact amount thereof being determined by the thermal expansion characteristics and the temperature change of the deposition mask itself.
The expansion of the deposition mask is known to have a thermal expansion effect which causes the well-defined pattern to be enlarged due to the expansion of the deposition mask at the operating temperatures of the deposition process.
Descriptions of the prior film deposition through masks and the methods and techniques for fabricating the same are disclosed in greater detail in a book entitled HANDBOOK OF THIN FILM TECHNOLOGY, Edited by Leon I. Maissel and Reinhard Glang, McGraw Hill Book Company, New York, N.Y., 1970, at Chapter 7 thereof entitled "Generation of Patterns in Thin Films" by Reinhard Glang and Lawrence V. Gregor at pages 7-1 to 7-10.
It is known in the art to produce electrical components and circuits from films through the generation of geometrical patterns in the film. A mask is typically used to form the pattern and the use of masks in a deposition process or in an etching process is described at pages 419 to 423, inclusive, in a reference entitled "THIN FILM TECHNOLOGY" by Robert W. Berry, Peter M. Hall and Murray T. Harris, Members of the Technical Staff, Bell Telephone Laboratories, Inc., published by D. VAN NOSTRAND COMPANY, INC. Also, the above reference "THIN FILM TECHNOLOGY" discloses at pages 451 to 453, inclusive, the use of a mechanical mask in a vacuum evaporation process to provide deposited layers in patterns which are within .+-.0.002 inches of the mechanical mask pattern with the capability of obtaining tolerances in the order of .+-.0.002 inches. At page 452 of the reference "THIN FILM TECHNOLOGY", a process is described which utilizes a deposited photoetched film positioned on a substrate as a deposition mask to produce a deposited pattern having a sharp edge definition.
One technique using a vapor deposition process including deposition through a deposition mask is disclosed in U.S. Pat. No. 3,867,368 to Lazzari. In fabrication of a thin film transducer having pole piece layers and one or more winding layers, any one of several techniques can be used in an attempt to precisely control the width, length, depth and registration of the various layers relative to the prior deposited and post deposited layers. The apparatus utilized in fabricating such thin film transducers relied solely on the use of mask-carriage assembly which functionally attempted to index and accurately position a specific deposition mask pattern relative to a substrate.
One known vapor deposition process which utilized a deposition mask having a circular aperture for evaporating an electrode of gold or silver in a small circular area which produced a sharply defined small central area of deposited material forming an electrode on a base coated electrode of a high frequency crystal to calibrate the same is disclosed in an article entitled "The Deposition of Electrode onto Crystal Vibrators" which appeared on pages 264 to 268, inclusive in a reference known as "Vacuum Deposition of Thin Films" by L. Holland, 1963, published by Chapman and Hall, Ltd., 37 Essex W.C.
In a process described in an article entitled "THINCO'S UNIQUE VACUUM DEPOSITION SYSTEM" which appeared at page 11 et seq in the June, 1980 issue of "r.f. design", a Cardiff Publication, electronic components were produced by sequential deposition of metal and dielectric films through aperture masks. As noted in the above identified article, a variety of masks with precision apertures and precision mask indexing were utilized. The articles included a FIG. 6 which illustrated one of six masks having a plurality of semi-circular apertures used for the fabrication of a thin film, a multi-layer inductor. The mask indexing and registration apparatus or the method for accomplishing the same was not disclosed. The article noted that how small the size of the apertures can be made with precision tolerances depends on several factors including the mask thickness. The sizes of aperture diameters obtained were in the order of 0.005 inches.
The use of photo resist material as a mask in an ion milling application is disclosed in U.S. Pat. No. 4,119,881. The ion beam source was produced by use of grids positioned in the form of a "truncated cone".
The use of electron-sensitive, two level resist to produce a mask for use in a vapor deposition process is also known in the art. In one known application, a Josephson junction was fabricated by use of a two-level, electron sensitive resist to produce a self-aligning deposition mask utilized in conjunction with an effective evaporation source. The use of electron-sensitive, two-level resist was disclosed in an article entitled "Submicron Tunnel Junction At Bell Labs", which appeared in the July, 1981 issue of "Semiconductor Industry" at pages 9 and 10.
In the above disclosed technique, a base electrode was evaporated at an oblique angle to a substrate through a pattern mask formed in the two-level resist. The deposition mask was fabricated using electron-lithography techniques with a two-level electron sensitive resist to produce a severely undercut lower support layer in one level of the resist, which was contiguous the substrate, to define an electrode deposition area on the substrate. The other upper level of the resist produced a second spaced, upper supported resist layer which overhangs the support layer and has a pattern formed therethrough adjacent the electrode deposition area. The upper resist layer formed a suspended resist span which defined a deposition mask having a specific pattern for a counter electrode. A base electrode of lead-indium (Pb In) alloy was evaporated at an oblique angle to the substrate on the side of the deposition mask opposite to the counter electrode section, through the patterned resist. The base electrode was then oxidized and a counter electrode of lead (Pb) was deposited from an oblique angle on the other side of the deposition mask having the counter electrode pattern. The counter electrode was deposited in an overlap pattern onto the base electrode. The resulting super conducting junctions were formed in the overlap areas of the Lead Indium-Oxide to Lead layers. The base electrode was less than 0.05 micrometers thick.